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  1. #1
    Junior Member
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    Sep 2002
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    Converting lenses from 35mm(film) to DSLR (Sony)

    Ok new to DSLR. Just got Sony a-100 camera that came with 18 - 70mm lens.

    Looking for telephoto. It would seem that my 70 mm DSLR equals my 35mm (film) 50mm lens, is that correct?

    So If I want a DSLR Telephoto i would need at least (digital)75 mm to 300 to yield a
    50 to 200 +/- 35mm (film) format lens since it would appear the format is different by 50%

    Any one care to reference why it isn't the same? This isn't metric to US units so let me know.

  2. #2
    Poster Formerly Known as Michael Fanelli mwfanelli's Avatar
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    Re: Converting lenses from 35mm(film) to DSLR (Sony)

    Quote Originally Posted by CremeStout
    Ok new to DSLR. Just got Sony a-100 camera that came with 18 - 70mm lens.

    Looking for telephoto. It would seem that my 70 mm DSLR equals my 35mm (film) 50mm lens, is that correct?

    So If I want a DSLR Telephoto i would need at least (digital)75 mm to 300 to yield a
    50 to 200 +/- 35mm (film) format lens since it would appear the format is different by 50%

    Any one care to reference why it isn't the same? This isn't metric to US units so let me know.
    The notion of "equivalent" for digital cameras is one of the worst things ever devised. No one talks about the "35mm equivalent" of 6x7 lenses or large format lenses. Anyway...

    The thing that changes is the field of view, the "left-to-right/diagonal" distance. The characteristics of the lens itself does not change. A 45mm lens on 35mm has a FOV that is "standard", on a 6x7 it's wide angle, on a 1.6 digital is a mild telephoto. But in ALL cases it has all the characteristics of a 45mm lens. This wide wide angle on a 6x7 camera has no where near the DOF or perspective of a wide angle on a 35mm camera.

    The lens will project the same image circle it aways has. The digital sensor just crops off the edges, the same thing you could do yourself in Photoshop. It does NOT become a telephoto lens any more than cropping in software creates a telephoto lens.

    There is no real "magnification." This is why a 50mm lens will not provide you with an 80mm look. It will be 50mm with the edges gone. Decide if you need the look and perspective of 75mm (then buy 75mm) or you just want the FOV of a 75mm (buy 50mm).

    Is that confusing enough? :-)
    "Sometimes I wonder whether the world is being run by smart people who are putting us on, or by imbeciles who really mean it." --Mark Twain

  3. #3
    May the force be with you Canuck935's Avatar
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    Re: Converting lenses from 35mm(film) to DSLR (Sony)

    Okay that was a bit confusing. Simply, take your 18-70mm kit lens for example. To figure out what your field of view will be equivalent to with your Sony a100, multiply the focal lengths by 1.5. So, that means your 18-70mm lens will give you the equivalent field of view of a 27-105mm lens on a 35mm format camera.

    The reason you need to do this math when using most DSLR's is because the bodies are designed to use the same lenses as 35mm, but the sensors in the DSLR's are actually smaller than a 35mm frame (most are the size of an APS frame).

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